1. Field of the Invention
The present invention relates to an apparatus for recording a two-dimensional image on heat-sensitive recording paper by controlling ON times of heating element dots of a thermal head.
2. Description of the Prior Art
Several techniques have been extensively studied to reproduce hard copy images such as photographs from electrical image signals derived from television cameras, video cameras, video disks and electronic still cameras. Among these, a typical example is a heat-sensitive image recording technique. According to this technique, a line thermal head having heating element dots (corresponding to one line) aligned in a horizontal direction is shifted on heat-sensitive recording paper (including heat-sensitive color development paper and a laminate of an imaging sheet and a thermal transfer ink sheet). Only predetermined heating element dots are energized and heated in accordance with input image signals, and pixels for one horizontal line are printed on the recording paper. The pixels in units of horizontal lines are sequentially printed to obtain a single two-dimensional image.
In this case, the number of heating element dots corresponding to one-line pixels is determined by an image size. For example, as many as 1280 dots are used for one-line pixels. However, when such a large number of dots is used, a maximum current flow for energizing all the dots is increased. As a result, a high power source is required.
In order to solve the above problem, a conventional system is proposed wherein a plurality of heating element dots are divided into, for example, two blocks which are then time-divisionally driven. According to this system, although print time for one-line pixels is doubled, a maximum current can be decreased to 1/2 or less.
A temperature distribution of one heating element during heating is illustrated by a curve having a peak at the center thereof, as shown in FIG. 1. Assuming that all the dots of each block are heated, a temperature of a boundary between an energized block B.sub.n (i.e., hatched dot regions) and a nonenergized block B.sub.n+1 (i.e., nonhatched dot regions) is low. The term "temperature" is abbreviated by TEM in the drawings. Since the region outside the boundary of the energized block B.sub.n belongs to the nonenergized block B.sub.n+1 the temperature is low, and heat escapes. This phenomenon cannot be prevented and occurs when the block B.sub.n is turned off and the block B.sub.n+1 is turned on. A temperature distribution shown in FIG. 2B is obtained. Therefore, a boundary region between two adjacent blocks cannot be sufficiently heated at the beginning and end of a heating cycle (FIG. 2C). The printed pixels are blanked at the boundary, thus resulting in white dots thereat. The white dots appear as a white line in an image along the feed direction of the recording paper. When an image such as a photograph is created, the white line results in a decisive disadvantage.